1. Technical Field
The present invention generally pertains to a method and apparatus for attaching a composite plastic vehicle body to a metal frame. More specifically, but without restriction to the particular embodiment and/or use which is shown and described for purposes of illustration, the present invention pertains to an arrangement for attaching a plastic vehicle body to a metal frame which incorporates a variety of fasteners and metal spacers in a predetermined spatial relation and a related method.
2. Discussion
The use of plastic materials in motor vehicles has increased greatly in the recent past and the trend continues. For example, many original equipment manufacturers are attempting to implement plastic panels where metal stampings were traditionally used. Due to the very high cost of stamping large metal panels, manufacturers have exhibited a special interest in the largest vehicle body panel, the floor panel.
Creating a motor vehicle floor panel from composite plastic materials poses a formidable design challenge for at least three reasons. Firstly, the vehicle body must be secured to a load bearing structure such as a traditional metal frame. Accordingly, a fastening arrangement capable of securely fixing the composite plastic floor panel to the frame without crushing or otherwise permanently damaging the composite plastic is required. In addition, a composite material phenomenon known as xe2x80x9ccreepxe2x80x9d must be addressed. When plastic creeps, the material slowly moves from areas of high compressive stress to areas of low stress. The threaded fasteners commonly used to mount vehicle bodies to undercarriages or frames are capable of very high compressive stress under the fastener head after proper installation. The clamping force generated by the fastener is directly proportional to the amount of fastener elongation induced at installation. Accordingly, any movement of composite plastic material away from the area beneath a fastener head reduces the fastener compressive load (elongation) and also significantly reduces the clamp load as initially applied because of the composite material creep or deformation under a compressive load.
Secondly, other structures such as seat frames and passenger seat belt brackets must be rigidly connected to the vehicle in order to ensure passenger safety by sustaining the dynamic loading under use. The use of a plastic vehicle body increases the difficulty of this task because plastics generally do not exhibit compressive strength, tensile strength or bending strength as great as steel or other suitable applied metals.
Lastly, the engineering composite plastic materials in use today tend to exhibit a coefficient of linear thermal expansion (CLTE) much greater than that of steel. Therefore, care must be taken to accommodate a greater change in panel size per change in temperature, especially where the plastic body panel and metal members meet.
Existing designs have attempted to address the issue of lower strengths and creep within highly loaded composite plastic panels. For example, one such design incorporates a fastener including a metal sleeve embedded in the composite plastic component to react the loads introduced by fasteners such as bolts and rivets. While this concept has had commercial success, other issues need to be addressed. Specifically, the thermal expansion associated with temperature changes within large plastic panels still poses a risk of buckling or other material or mechanical failures. In an attempt to implement as many plastic components as possible while maintaining structural integrity, manufacturers have limited the length of the plastic panels in order to limit the associated thermal expansion differential. While this method has been effective, a patchwork of smaller panels is undesirable compared to the advantages obtained by utilizing one homogenous body panel such as a roof, a floor panel, door panels, or outer/inner body panels.
Accordingly, it is a general object of the present invention to provide an arrangement for attaching a plastic vehicle body to a metal frame which overcomes the disadvantages of the prior art.
It is another object of the present invention to provide a plastic vehicle body attachment system capable of interconnecting a variety of components exhibiting different coefficients of linear thermal expansion.
It is yet another object of the present invention to manage the amount of expansion at selected mechanical interfaces without degrading the overall structural load carrying abilities of each component of the system.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.
In one form, the present invention provides a plastic vehicle body attachment system for a motor vehicle. The attachment system includes a metal support, a plastic panel having an aperture, and a fastener. The fastener is disposed in the aperture and detachably couples the plastic panel to the metal support. The fastener is smaller than the aperture by an amount that varies in relation to the distance between the fastener and a fixed point on the composite panel such that the size of the aperture increases as the distance between the fastener and the fixed point increases.
In another form, the present invention provides a method of securing a first panel to a second panel including the following general steps:
determining a first coefficient of thermal expansion of said first panel;
determining a second coefficient of thermal expansion of said second panel;
measuring a distance from a reference point to an attachment point;
providing a fastener;
providing an aperture in said first panel;
determining a clearance between said fastener and said aperture which varies according to said distance; and
passing said fastener through said aperture to secure said first panel to said second panel.